Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials, which is becoming more and more popular in plastic industry. In response to the quest for advanced and diversified product design and the thend for producing smaller, lighter and more delicate products with satisfactiry aesthetic and functional results, there are a diversity of plastic injetion molding processes being developed. Especially for producing exterior parts for automobiles, electronic devices or household eletric appliances that are generally large in volume but should still be built the lighter the better while maintaining good mechanical strength and high dimension stability, there are two advance injection molding techniques being developed today, which are the gas-assisted injection molding (GAIM) process and the MuCell process.
The MuCell Process involves the highly controlled use of gas in its supercritical state (SCF) to create millions of micron-sized voids in molded parts to make a foamed part, that is commercially developed by Trexel Co. Ltd. under worldwide exclusive license from the Massachusetts Institute of Technology (MIT). The MuCell process, being a plastic foaming process, is designed to produce microcellular plastic foams by mechanically or chemically dispersing an inert gas, usually carbon dioxide (CO2) or nitrogen (N2) that is used as foaming agent, into the polymer melt. Accordingly, the MuCell process is favored by its capability of performing in a high cell nucleation rate within the foaming material to create foams with evenly distributed and uniformly sized microscopic cells. In addition, since Earth's atmosphere is rich in carbon dioxide and nitrogen that they are easy to obtain, the cost of the MuCell process is comparatively less expensive, not to mention that the consumption of plastic material for producing form materials in this MuCell process can be reduced by more than 30%. Moreover, foam materials produced by this process offer improved consistency and homogeneity of cell structures, which can result in products with superior mechanical properties compared to other foaming systems. Thus, the MuCell technology not only offers reduction in manufacturing cost, but also can provide better product quality.
Despite of the aforesaid advantages, the products produced by the MuCell process generally have poor surface appearance. For overcoming such shortcoming, a gas counter pressure technology (GCP) was developed and widely applied in industry after year 1980.
In an injection molding method disclosed in Japanese Patent Application Laid-Open No. 2004-223879 by Asahi Kasei Cooperation, carbon dioxide is used as a counter pressure gas that is being injected into a mold cavity for pressurizing the same to a specific pressure prior to the filling of a foamable resin into the mold cavity, and thereby, not only the gas at the specific pressure will dissolve in the resin so as to reduce the melt viscosity of the resin, but also if the specific pressure is higher than the foaming pressure of the resin, the foaming of the resin will be suppressed, and if the specific pressure is lower than the foaming pressure of the resin, it can be used to produce products with evenly distributed microscopic cells but with different foam sizes by subjecting the resin to different counter pressures, while simultaneously improving the surface appearance and shrinkage/warpage of the resulting products. The GCP for injection molding process is operated as following:                (1) prior to the starting of a molding cycle, filling the mold cavity with a specific gas under an adequate pressure, whereas the specific gas can be an inert gas whichever is not going to react with the melt resin;        (2) filling the melt resin into the mold cavity for subjecting the flowing resin to a stable counter pressure;        (3) using a pressure control unit arranged at the outlet of the mold cavity for adjusting and thus maintaining the pressure stability of the mold cavity until the injection of the melt resin is completed; and        (4) retrieving the molded resin after cooling.        
For the aforesaid injection molding process using GCP, a runner is required to be formed in the mold for connecting the mold cavity with outside world, so that leakage prevention mechanism as well as seals are required to be constructed near the whereabouts of the runner.
Because of the packing pressure effect working on the flow front of the melt resin by the counter pressure, the use of GCP for improving surface appearance as well as the shrinkage and warpage in the foamed products is becoming more and more popular. For example, in Year 2004, Ohshima uses carbon dioxide as counterpressure gas in a gas counter pressure (GCP) method for injection molding resins of low-density polyethylene (LDPE) and polypropylene (PP), in that by examining the molecular weight relating to the flow front of the melt resin, it is discovered that the gas is absorbed into the flow front of the flowing resin or enters into the interface between the mold and the resin and is dissolved in the surface layer of the resin. Moreover, in a PC/SCN2 microcellular foaming process using GCP by Andrzej, et al., the surface roughness of an injection-molded tensile sample is reduce from 23.11 μm to 0.85 μm with even more evenly distributed foams being formed therein but at the cost that its weight reduction ratio is decreased from 12.8% to 10.2%. In addition, at Year 2006, Michaeli and Cramer overcome the surface defecting problem that is commonly seen in 2 microcellular foaming process by applying a gas counter pressure method in a MuCell process.
In addition, in a high-speed chemical foam injection molding method using gas counter pressure that is disclosed in CN 101007437A, after injecting a gas into a mold cavity of a closed mold for pressurizing the same, a melt resin is fed into the mold cavity for creating foamed resin filled with microcellular foams while enabling the surface of the foamed resign to abut exactly against the walls of the mold cavity. Thereafter, after the mold is cooled and the foamed resin inside the mold cavity is solidified and molded, the mold can be opened for retrieving a plastic part with glassy surface and evenly distributed microcellular foams. It is noted that the technique disclosed in the aforesaid Chinese patent is similar to the one disclosed in JPA 2004-223879.
For improving the surface appearance and quality of products produced by MuCell process, after experimenting repetitively, it is noted that by the use of counterpressure gas, not only surface quality can be improved, but also the size of forms to be established can be controlled as well as the distribution evenness thereof can be enhanced. Moreover, the proper control to the mold temperature is also helpful for surface quality improvement.
Therefore, it is in need of an apparatus for controlling gas counterpressure and mold temperature in a supercritical fluid (SCF) microcellular injection molding process for achieving better surface quality improvement and foam size control.